1. Field of the Invention
The present invention is generally directed toward waveform shaping for input/output devices. More specifically, the present invention relates to controllably enhancing slew of a driver to shape an output signal of the driver.
2. Discussion of Related Art
Electronic devices usually exchange signals over a bus structure. A transmitting device applies (or drives) signals onto a bus signal path using an output signal driver circuit and a receiving device receives that signal through a input signal receiver circuit. As the applied signal electronically (or optically) transfers over the medium of the interconnecting bus, some signal distortion may occur. Often, such distortion may start at the signal generated by the output driver circuit and further deteriorates as the signal traverses the bus structure signal pathway.
One common bus structure for connecting peripheral I/O devices to a computer system is the Universal Serial Bus. Universal Serial Bus (USB) is a common standard in the computer industry for transferring data between computers and peripheral devices. The most widely used USB standards, such as the USB Specification 1.1 and the USB Specification 2.0, allow for data to be transferred at rates much faster than those previously achieved with typical serial or parallel ports of a personal computer. USB also provides a simple, serial data bus capable of communicating with a wide variety of devices while simplifying cabling requirements in a system. USB further provides power and “hot swapping” capabilities to many peripherals. Hot swapping is the ability to add and remove devices connected to a computer while the computer is running such that the operating system of the computer automatically recognizes the change. For example, with a USB connection, a digital camcorder can transfer digital video data from the camcorder to a computer simply by connecting the camcorder to the computer via a USB cable. The operating system automatically detects the USB connection to the camcorder and optionally supplies power to the camcorder. The digital video data is then transferred to the computer via the USB cable at the user's discretion.
The USB standards, at present, define a “low speed” transfer with signals clocked at about 1 Mhz, a “full speed” transfer with signals clocked at about 12 Mhz and a “high speed” transfer with signals clocked at about 480 Mhz. The low and full speed transfer signaling protocols are similar while the high speed signaling protocol and associated circuits adhere to a substantially different signal timing standard. While transfer speed is an important concern when designing such devices, signal quality is essential to maintaining information integrity as the data is transferred over the interconnecting bus structure. Circuit designers often implement complicated and expensive circuits to ensure that the information integrity is maintained or in the alternative to ensure that incorrect data can be detected and corrected. It is preferable to design a driver circuit that ensures data integrity at its source—namely at the output signal driver. In other words, the shape of a signal as generated at its source can be an important factor in data integrity problems. Further, a poorly generated signal shape can be further degraded as it is transmitted over the conductive signal paths of the interconnecting bus structure.
A common approach to assure quality signal generation is to control the “gain” of the signal representing the data as applied to an output driver. A typical output driver (as for example in USB) includes an operational amplifier coupled to the gate of an output transistor. The output transistor couples an appropriate power supply “rail” to the output signal. The output signal is typically applied to a conductive “pad” for interconnection to the associated bus structure. The operational amplifier receives a digital input signal and increases the gain of that signal by driving the gate of the output transistor such that the transistor conducts more current (i.e., increased current flow through its source and drain) to an output conductive pad than would direct application of the input signal to the output pad. Control over the increased gain generated by the operational amplifier is achieved with a feedback of the output signal to the amplifier through a capacitor. This feedback structure stabilizes the output signal and hence “controls” the output signal shape.
However, present output drivers have failed in delivering “high” quality signals while maintaining lower power requirements. A problem in delivering a high quality signal from the typical output driver resides in the driver's inadequacy to quickly drive the output transistor. The operational amplifier drives the output transistor such that the operational amplifier maintains control of the output signal. Unfortunately, the “slew rate” of the operational amplifier is slow in driving the gate of the output transistor. The slew rate is the rate of change of the output voltage of the operational amplifier when an input signal is applied to its input. In other words, when a voltage is applied to an input, the operational amplifier will respond by generating an output drive signal to the gate of the output transistor that changes (rises or falls) over some period of time. If the gate voltage of the output transistor rises too slowly, the signal on the output pad may briefly float uncontrolled and hence provide a poorly shaped signal on the output pad. This brief uncontrolled time for the slew of the output signal results in a poor signal shape at the output pad. Increasing the slew rate of the signal applied to the gate of the output transistor improves the shape of the output signal of the driver as applied to the output pad by reducing the time that the signal on the output pad floats uncontrolled.
The slew rate of the signal applied to the output transistor gate is, at present, a function of the operational amplifier design and the capacitive feedback circuit used to control the operational amplifier output signal. The slew rate of the feedback controlled operational amplifier is approximately inversely proportional to the size of the capacitor used in the feedback to the operational amplifier. For example, when an input signal is applied to the operational amplifier, the output signal will respond at a rate substantially determined by the size of the capacitor. Thus, to change the slew rate, one could simply change the size of the capacitor. However, such a change to the capacitor affects the stability and power consumption of the output driver.
Some approaches attempt to improve this condition by increasing the Direct Current (DC) of the operational amplifier to thereby increase its slew rate. This approach allows the amplifier to quickly pull its output to a level that quickly restores the feedback loop to its desired operating level. These approaches may increase power requirements due to large Direct Current (DC) bias currents within the operational amplifier; they may also cause instability problems because of poor settling performance and ringing because of excessive bandwidth of the operational amplifier.
Other approaches include clamping the control terminal of the output transistor to a voltage that turns on the transistor. These approaches eliminate the need for a large slewing current but are dangerous because the output transistor will remain undesirably driven at all times. Each of these solutions has unwanted effects that undermine the goals of providing a high quality output signal.
These problems and prior solutions are typical of USB output driver circuits but are also common in other signaling standards and protocols.
It is evident from the above discussion that there is a need to provide a simple output driver that maintains control of the slew of an output signal without consuming inordinate amounts of power.